How to get normal/displacement/ao maps for a part of a photo
This shouldn't bother you much, since you probably have the original cut image, with all the removed parts transparent (?)

In that case you can use the alpha to either make a mask / selection and remove the useless bits from the generated maps in photoshop / gimp,

Or you can use the alpha from the cut color map in color mix nodes in the node editor. Either way it won't have any effect.